THE VARIATION CHARACTER OF SEISMIC VELOCITY RATIO IN SOURCE REGION AND FIELD AREA BEFORE AND AFTER ZHANGBEI M_S6.2 EARTHQUAKE

The seismic velocity ratio for earthquakes (M_L≥2.0)during 1991 to 1999 in the region 112°30'~116°00'E,39°00'~42°00'N is calculated.The variation character in time and space and the differences of the seismic velocity ratios before and after Zhangbei M_s6.2 earthquake in source region and field area are analyzed.The results show that distinct low value anomalies of seismic velocity ratio in time distribution appeared in source region and field area before the event.The course is decline-low value-quietude-recovery-occurrence.The low value anomalies are concentrative and synchronous in spatial distribution.The calculation is tested with T-test.The results indicates that the seismic velocity ratio anomalies in source region and field area are very obvious before Zhangbei M_s6.2 earthquake.The seismic velocity ratio in field area sustained low value after great changes since 1995.The seismic velocity ratio became uniform and its time and space variation also became normal after the event.